FIELD OF THE INVENTION
The present invention relates to a bandgap reference voltage source with a temperature-compensating operational amplifier. The operational amplifier has a first input stage at which the potential of a first node is available, and a second input stage at which the potential of a second node is available. The invention also pertains to a method for operating such a bandgap reference voltage source.
Reference voltage sources of that type and their operation have been known for a long time. The principle behind these reference voltage sources is based on the fact that the base-emitter voltage of a bipolar transistor (or the forward voltage of a diode) is used as voltage reference. However, that voltage reference, the value of which is designated by V.sub.BE below, depends on the temperature to a relatively great extent. For example, at 0.6 V the temperature coefficient is approximately -2 mV/K. The temperature dependence can be compensated for if an appropriate temperature compensation voltage, designated as KV.sub.t below, is added to the voltage reference V.sub.BE. This temperature compensation voltage is preferably generated and processed with the assistance of a second transistor and an operational amplifier.
The fundamental structure of such a configuration will now be explained with reference to FIG. 3.
The circuit of FIG. 3 comprises (bipolar) transistors T1 and T2, resistors R1, R2 and R3 and an operational amplifier OP. The interconnections of the elements are as shown.
The reference voltage V.sub.ref generated by the configuration is EQU V.sub.ref =V.sub.BE2 +KV.sub.t
where ##EQU1## and where A.sub.E1 is the area of the emitter of the transistor T1 and A.sub.E2 is the area of the emitter of the transistor T2.
Given suitable dimensioning of the individual components in the configuration according to FIG. 3, it is possible to make the temperature dependence of the reference voltage completely disappear.
In practice, however, it turns out to be considerably more complicated to generate the reference voltage V.sub.ref than may be presumed from the above simplified explanation. The virtually unavoidable offset of the operational amplifier is primarily responsible for this.
The offset of an operational amplifier is a known effect which arises when the operational amplifier has an asymmetrical structure, in other words, for example, when the input transistors of the operational amplifier have different sizes or the threshold voltages of these transistors are different; it acts like an (offset) voltage source connected upstream of one of the input terminals of the operational amplifier.
Due to the presence of the offset (of the offset voltage source), the reference voltage V.sub.ref generated by the configuration according to FIG. 3 becomes ##EQU2##
Reference is had, with regard to this relationship, to P. E. Allen and D. R. Hollberg, "CMOS Analog Design", Saunders College Pub., 1987, pp. 590-596.
It is evident from the above formula, that the reference voltage is also influenced by the offset of the operational amplifier; the voltage V.sub.os of the equivalently introduced offset voltage source is present, in fact amplified by the factor 1+R2/R1, at the output of the operational amplifier.
Given customary dimensions an emitter area ratio of 10:1 is used for the transistors, as a result of which 1+R2/R1 must become approximately 10.5 if the temperature dependence of the reference voltage is to be eliminated (cf. Allen et al., above).
If an implementation of the bandgap reference voltage source in CMOS technology is presumed, and the offset of up to 30 mV which is customary in such cases, then the reference voltage, which normally amounts to about 1.2 V, would be corrupted by up to 0.315 V by the offset of the operational amplifier.
If the variation of the absolute value of the reference voltage is to be kept low, the offset must be compensated for.
For this purpose, provision could be made, for example, to trim the configuration individually in such a way that the respective offset is made to disappear. On account of the temperature dependence of the offset, however, such compensation is effective only at a quite specific temperature.
Another option in compensating for the offset consists in measuring the offset from time to time and correspondingly charging a capacitor which compensates for the offset. However, during the measurement and charging operation, the operational amplifier cannot be used as intended, as a result of which the reference voltage output by the operational amplifier can be output only with interruptions of greater or lesser duration. These interruptions occur repeatedly because the measurement and charging operation is necessary cyclically, in particular on account of discharge operations at the capacitor and on account of the temperature dependence of the offset. If a reference voltage is required which is available permanently without any interruptions, then such offset compensation is unsuitable or in any event not optimally suitable.
Another general discussion of bandgap reference voltage sources can be found for example in U. Tietze and Ch. Schenk, "Electronic Circuits: Design and Applications". Springer, Berlin, 1990; pp. 499-501 [cf. Halbleiterschaltungstechnik, 10.sup.th Ed., 1994, pp. 558ff]. There, an operational amplifier is used for comparing two voltages which are proportional to the base-emitter voltages of two bipolar transistors. Another bandgap voltage reference is discussed in the European patent disclosure EP 0 661 616, where the voltage reference generated is fed to an amplifier having a high gain factor. The signal amplified in this way is fed to a voltage regulator which regulates the supply voltage of the circuit for the purpose of generating the bandgap voltage reference. Finally, German patent DE 41 24 427 describes a bandgap reference voltage source in which an improved independence with respect to temperature influences is achieved by setting a channel resistance ratio.